ACT4455
Rev 2, 21-Nov-12
STABILITY COMPENSATION
Figure 2:
Stability Compensation
If R
COMP
is limited to 15kΩ, then the actual cross
over frequency is 6.36 / (V
OUT
C
OUT
). Therefore:
C
COMP
=
6.67
×
10
−
6
V
OUT
C
OUT
(F)
(15)
STEP 3. If the output capacitor’s ESR is high
enough to cause a zero at lower than 4 times the
cross over frequency, an additional compensation
capacitor C
COMP2
is required. The condition for using
C
COMP2
is:
: C
COMP2
is needed only for high ESR output capacitor
The feedback loop of the IC is stabilized by the
components at the COMP pin, as shown in Figure
2. The DC loop gain of the system is determined by
the following equation:
A
VDC
=
0 . 808 V
A
VEA
G
COMP
I
OUT
⎛
1.1
×
10
−
6
⎞
R
ESRCOUT
≥
Min
⎜
,0.012
×
V
OUT
⎟
⎜
C
⎟
OUT
⎝
⎠
(Ω)
(16)
And the proper value for C
COMP2
is:
(8)
C
COMP 2
=
C
OUT
R
ESRCOUT
R
COMP
(17)
The dominant pole P1 is due to C
COMP
:
f
P1
=
G
EA
2
π
A
VEA
C
COMP
I
OUT
2
π
V
OUT
C
OUT
(9)
Though C
COMP2
is unnecessary when the output
capacitor has sufficiently low ESR, a small value
C
COMP2
such as 100pF may improve stability against
PCB layout parasitic effects.
Table 1 shows some calculated results based on
the compensation method above.
Table 1:
Typical Compensation for Different Output
Voltages and Output Capacitors
V
OUT
2.5V
3.3V
5V
2.5V
3.3V
5V
The second pole P2 is the output pole:
f
P 2
=
(10)
The first zero Z1 is due to R
COMP
and C
COMP
:
1
f
Z 1
=
2
π
R
COMP
C
COMP1
(11)
And finally, the third pole is due to R
COMP
and
C
COMP2
(if C
COMP2
is used):
f
P 3
=
1
2
π
R
COMP
C
COMP2
C
OUT
47μF SP CAP
47μF SP CAP
47μF SP CAP
680μF/6.3V/30mΩ
680μF/6.3V/30mΩ
680μF/6.3V/30mΩ
R
COMP
5.6kΩ
7.5kΩ
11kΩ
15kΩ
15kΩ
15kΩ
C
COMP
C
COMP2
5.6nF
4.7nF
3.3nF
3.3nF
3.3nF
4.7nF
None
None
None
220pF
220pF
220pF
(12)
The following steps should be used to compensate
the IC:
STEP 1. Set the cross over frequency at 1/10 of the
switching frequency via R
COMP
:
: C
COMP2
is needed for high ESR output capacitor.
R
COMP
=
2
π
V
OUT
C
OUT
f
SW
10 G
EA
G
COMP
×
0 .808 V
(13)
Output Cable Resistance Compensation
To compensate for resistive voltage drop across the
charger's output cable, the ACT4455 integrates a
simple, user-programmable cable voltage drop
compensation using the impedance at the FB pin.
Use the curve in Figure 3 to choose the proper
feedback resistance values for cable compensation.
R
FB1
is the high side resistor of voltage divider.
=
0 . 48
×
10
8
V
OUT
C
OUT
(Ω)
STEP 2. Set the zero f
Z1
at 1/4 of the cross over
frequency. If R
COMP
is less than 15kΩ, the equation
for C
COMP
is:
C
COMP
=
3 . 18
×
10
R
COMP
−
5
(F)
(14)
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